Regenerative chemical absorption process is widely
used in industry for removal of carbon dioxide
from natural gas, with activated aqueous solutions
of alkanolamines considered
a beneficial blend. One of the most promising
blends is the aqueous mixture of
methyldiethanolamine (MDEA) activated by small
amount of piperazine, due to the fairly high rate
of reaction of piperazine as a cyclic diamine with
carbon dioxide and the low heat of regeneration of
MDEA as a tertiary alkanolamine. However,
mechanisms involved in absorption of carbon
dioxide by means of aqueous piperazine activated
MDEA solution are not well understood and
available knowledge within the open literature is
limited.
This thesis aimed to extend our knowledge of the
topic by providing insights into the
intermolecular interactions involved in mechanisms
governing the absorption of CO_2.
Furthermore, understanding of the processes
associated with the undesired trapping of methane
was an additional objective. Characterization of
the blend to analyze the distribution of the
absorbent and absorbate molecules inside the amine
solution was yet another goal for this work. In
general, a better understanding of intermolecular
interactions and correlations between molecular
constituents of the amine solution during the
overall process of carbon dioxide absorption from
natural gas was expected from this work.
To achieve this purpose, MD simulation was chosen
as the scientific method to study the system on
the molecular scale but prior to starting the
simulation, the following had to be accomplished.
Initially, an extensive literature review was
carried out to determine the speciation of the
solution in thermodynamic conditions relevant for
absorption units in amine gas treating plants.
Moreover, the natural gas was simplified to an
impurity free binary mixture of methane and carbon
dioxide during the simulation.
In order to study the system on the molecular
level, we obtained missing force-field parameters
by combining the OPLS parameter with ab-initio
calculations for piperazine molecule, piperazine
carbamate, protonated piperazine and molecular
MDEA. To investigate the most important torsional
angles in MDEA and piperazine carbamate, geometry
of the molecules was optimized and rigid
coordinate scan was performed to define true
torsional angles, so that ultimately, new
functions for dihedral angles of these molecules
were derived. In order to calculate the partial
atomic charges for all the above mentioned models,
Mulliken, Löwdin and CM4 partial atomic charges
were computed using 6-31G** basis set and SM8
solvation model. Given the reliability and
accuracy of the CM4 charges, they were chosen to
be implemented into the final models. Other models
for carbon dioxide, methane and water molecules
needed for the simulations were taken from
literature.
Constraint MD simulation was performed at constant
temperature using MDynaMix Ver.5.1 for one primary
and four reference systems.
It was observed that the MDEA was separated from
aqueous phase and positioned alongside the gas-
liquid interface. Moreover, it was determined that
piperazine molecules were spread within MDEA
phase, while protonated piperazine and piperazine
carbamate mainly dissolved in the water bulk.
Hydrogen bond network inside the solution was...